1. Field of the Invention
This invention relates to a lighting device or a starting device for a high-voltage discharge lamp such as a metal halide lamp. Here, the lighting device or the starting device is of the type using an alternating current (AC) or a rectangular-wave current.
2. Description of the Prior Art
It is known to light or start a high-voltage discharge lamp such as a metal halide lamp by using an alternating current (AC). In the case where such a discharge lamp in a cold state is started, the intensity of light generated by the discharge lamp remains low until the pressure of a metal vapor therein rises to a sufficient level.
Japanese published unexamined patent application 5-144577 corresponding to U.S. Pat. No. 5,365,152 discloses a discharge-lamp lighting apparatus including a lighting control circuit. During a time of starting the discharge lamp, the lighting control circuit serves to control its output in response to a length of the last off-time and an on-time after the start of discharging. In the discharge-lamp lighting apparatus of Japanese application 5-144577, a power supplied to the discharge lamp is controlled in response to the output of the lighting control circuit.
Some of the prior-art AC-based lighting devices for high-voltage discharge lamps have a problem as follows. Immediately after a high-voltage discharge lamp in a cold state is started by using AC power, the discharge lamp tends to go out upon a change of the polarity or direction of a lamp current between a positive side and a negative side.